1. Field of the Invention
The present invention relates to a wireless data receiver, and more particularly to a receiver that receives signals modulated in a multiband orthogonal frequency-division multiplexing (MB-OFDM) method.
2. Description of Related Art
An orthogonal frequency-division multiplexing (OFDM) modulation method has recently attracted attention as a technique for realizing high speed wireless data transmission. There is a standard on a wireless transmission method employing a combination of the OFDM modulation system and frequency hopping for ultra wideband (UWB) communication, that is, MB-OFDM method (ISO/IEC 26907 High Rate Ultra Wideband PHY and MAC Standard).
In the OFDM modulation method, each of a plurality of data included in one symbol is divided into a large number of subcarriers (multicarriers). A set of data to be sent all together are called a symbol (or OFDM symbol) In the OFDM modulation method, using inverse Fourier, a plurality of subcarriers are transformed into a signal occupying a time region. The signal is modulated with a carrier wave and transmitted. The subcarriers are arranged equidistantly, while maintaining orthogonality, on a frequency axis.
An MB-OFDM system is known as a communication method employing the OFDM modulation system for ultra wideband (UWB) communication. In the MB-OFDM system, a frequency band of 528 MHz is called a band, and a bundle of a plurality of bands (in principle, 3 bands; 2 bands as an exception) is called a band group. In the MB-OFDM system, communication is performed while changing the central frequency of a carrier wave (performing hopping) for each single OFDM symbol or a plurality of symbols in one band, so that a band occupied by the subcarriers is changed.
More specifically, when each bands of one band group are denoted as band 1, band 2, and band 3, data is transmitted while changing the band occupied by the subcarrier. For each single OFDM symbol, the band occupied by the subcarriers to transmit the data is changed in following order: band 1→band 2→band 3→band 1 . . . . The operation of transmitting data with changing the band occupied by the subcarrier for each OFDM symbol is called frequency hopping.
FIG. 18 shows how data transmission is performed with hopping the carrier frequency in the MB-OFDM system. As shown in FIG. 18, a piconet A and a piconet B performing communication by using three bands 1, 2, and 3 are located close to each other. The piconet as referred to herein is a network configured by a master (host) and a slave (device). The piconet A has a hopping pattern that ascends obliquely to the right in the figure in the order of band 1→band 2→band 3 . . . , whereas the piconet B has a hopping pattern that descends obliquely to the right in the order of band 3→band 2→band 1 . . . . As shown in FIG. 18, in order to obtain a diversity effect on the receiver side, data of the same contents are diffused into two conjugate symbols and sent sequentially into a transmission channel. More specifically, one symbol A1 is transmitted consecutively in two cycles as two conjugate symbols (A1-1) and (A1-2).
Where the piconet A and piconet B perform communication by using the same frequency band (band 2) at the same time instant, as at a time T1 or time T2, the two symbols (A1-2) and (B1-2) undergo frame collision, and symbols (A1-2) and (B1-2) interfere with each other. As a result, a problem arising in an environment in which the piconet A and piconet B are close to each other is that quality of the received symbol is degraded. The symbol quality as referred to herein, for example, means the amount of noise contained in the symbol. Thus, where interference occurs because the symbol (A1-2) and symbol (B1-2) use the same band, the amount of noise contained in the symbol (A1-2) and symbol (B1-2) increases. Such interference between the piconets is called adjacent piconet interference.
Japanese Patent Application No. 2005-269392 discloses a receiver configured so as to compensate the degradation of symbols caused by such adjacent piconet interference. FIG. 19 is a block diagram illustrating a symbol combination circuit that combines symbols of the receiver described in Japanese Patent Application No. 2005-269392. A signal to noise ratio (SNR) measurement unit 101 measures signal quality of time-diffused symbols. In a weight determination circuit 102, a weighting factor W1 of a first symbol (A1-2) is set based on a signal quality (SNR1) of the first symbol (A1-1), and a weighting factor W2 of a second symbol (A1-2) is set based on a signal quality (SNR2) of the second symbol (A1-2) The first symbol (A1-2) and second symbol (A1-2) have the same content data. An SNR is a value representing the ratio of noise contained in the signal.
An adder 103 generates a combined symbol as received data by adding up a value obtained by multiplying the first symbol (A1-1) by the weighting factor W1 in a multiplier 104 and a value obtained by multiplying the second symbol (A1-2) by the weighting factor W2 in a multiplier 105. By thus determining the weighting factors W1, W2 correspondingly to signal quality of each symbol, it is possible, for example, to set a small weighting factor W2 for a symbol (A1-2) with poor signal quality and set a large weighting factor W1 for a symbol (A1-1) with good signal quality, thereby making it possible to reduce the effect of signal quality degradation caused by adjacent piconet interference.
Japanese Patent Application No. 2005-6116 discloses a combination method employing a receiver of a spatial diversity system in which single MB-OFDM symbols transmitted at the same timing are received by two antennas. With this method, the weighting factor of the symbol received by the first antenna and the weighting factor of the symbol received by the second antenna are set correspondingly to the signal quality of a subcarrier.
However, in the OFDM modulation system, the effects produced by the interference or noise on each subcarrier are not uniform. As a result, where the weighting factor is set for each symbol as in the receiver described in Japanese Patent Application No. 2005-269392, it will be impossible to generate an optimum combined symbol under a frequency selective fading environment in which signal quality changes for each frequency. This problem will be described below in greater detail with reference to FIG. 20.
FIG. 20 shows a normalized electric field intensity E (dB) (referred to hereinafter simply as “electric field intensity”) of a plurality of subcarriers occupying the bands 1 to 3. In FIG. 20, a solid line represents an electric field intensity of a subcarrier occupying band 1, a dash-dot line represents an electric field intensity of a subcarrier occupying band 2, and a dot line represents an electric field intensity of a subcarrier occupying band 3.
A plurality of subcarriers are arranged equidistantly on a frequency axis in each band. More specifically, where a frequency gap between the subcarriers is denoted by m and a central frequency in each band is denoted by fn, the subcarriers are arranged in the order of . . . fn−2m, fn−1m, fn, fn+1m, fn+2m . . . . The electric field intensity representing the decrement of subcarrier amplitude differs between the bands. In FIG. 20, an electric field intensity of a subcarrier occupying a band with a central frequency f1, an electric field intensity of a subcarrier occupying a band with a central frequency f2, and an electric field intensity of a subcarrier occupying a band with a central frequency f3 are overlapped. In FIG. 20, the amplitudes attenuation received for each carrier in the bands is displayed by overlapping the central frequency of respective band. In other words, a region from the left end to the right end of a frequency axis in FIG. 20 is a frequency band of one band, and the frequency bands of bands 1 to 3 overlap at the central frequency on. The electric field intensity at a frequency f1+m of band 1, the electric field intensity at a frequency f2+m of band 2, and the electric field intensity at a frequency f3+m of band 3 are all shown as points on the frequency fn+m.
In a communication system with time diversity, the symbol is transmitted twice, while hopping the bands 1 to 3. Data of the symbol are transmitted by subcarriers occupying different bands on the same frequency shown in FIG. 20. The first symbol (A1-1) representing data contained in symbol A1 is transmitted by a subcarrier occupying band 1, and the second symbol (A1-2) representing data contained in the symbol A1 is transmitted by a subcarrier occupying band 2. Data constituting symbol A1 is transmitted twice with division between a subcarrier with a frequency f1+m and a subcarrier with a frequency f2+m.
The electric field intensity plotted against the ordinate does not depend on the contents of carried data. Therefore, the electric field intensity of the subcarrier can be estimated as a signal quality of the subcarrier itself. The electric field intensity is a power received by the receiver, and the estimation corresponds to good or poor signal quality when the electric field intensity is high or low, respectively.
In the conventional receiver (for example, see Japanese Patent Application No. 2005-269392), weighting factors of symbols are set based on signal quality of each symbol. In the conventional receiver, signal quality of the entire subcarrier carrying the symbol is determined for each symbol, and then weighting of the symbols is performed based on the signal quality of each symbol. For example, let us assume that the signal quality of the entire subcarrier occupying the band 1 is determined as “good’ and the signal quality of the entire subcarrier occupying the band 2 is determined as “poor”. In this case, a large weighting factor is set for the first symbol carried by the band 1 for which the signal quality is “good’, and a small weighting factor is set for the second symbol carried by the band 2 for which the signal quality is “poor”.